Tire wear contributes greatly to automobile and truck operating expenses. Wheel alignment is a very substantial factor in tire wear. Relatively small maladjustments in alignment can easily result in a high rate of tire wear, thus greatly increasing the motor vehicle operating costs.
In addition, bearing wear can make proper alignment, as hereinafter described in greater detail, difficult or impossible with resultant abnormal tire wear.
The alignment problem is complicated because of the use of steering gear which converts rotational steering wheel movement into side-to-side swiveling of the steerable wheels. As discussed in detail in applicant's co-pending U.S patent application, Ser. No. 157,194, filed June 6, 1980 (now U.S. Pat. No. 4,353,568), conventional motor vehicle steering gear typically includes a steering column to which a driver operated steering wheel is attached and in lower regions of which a projecting arm, known as a pitman arm, is connected to a gear box to swing, generally, from fore-to-aft, as the steering wheel is turned.
At each steerable wheel, a "knuckle" assembly is provided which includes a wheel mounting spindle, means for attaching the assembly to the vehicle's suspension system, and a projecting steering knuckle arm or lever enabling swiveling of the knuckle, and hence the wheel, for steering purposes. Two tie rods, usually of equal length, are provided, each being pivotally connected, at a tie rod ball and socket end, to a corresponding one of the steering knuckle arms. Opposite ends of the two tie rods are typically connected, in laterally spaced apart relationship, to intermediate regions of a transverse relay rod, one end of which is pivotally connected to the pitman arm. The other end of the relay rod is pivotally connected to the vehicle frame. Fore-to-aft pivotal movement of the pitman arm, as the steering wheel is turned, is transmitted through the relay rod to the individual tie rods which, through the steering knuckle arms, cause corresponding side-to-side swiveling or steering of the steerable wheels. Typical modern steering systems provide substantial tolerances throughout.
Vehicle steering control is typically provided by various static, angular adjustments of the steerable wheels. Most familiar of these wheel adjustments are caster, camber, steering axis inclination, toe-in, and toe-out in turn (also known as turning radius). Descriptions of these factors may be found in the book entitled Automotive Suspensions, Steering Alignment and Brakes by Billiet and Alley (n.p.: American Technical Society, 5th ed. 1974). Overall alignment is also dependent upon the relationships of the various sets of wheels. It is most important that there be no lateral offset between the sets of wheels (i.e., they should "track"), and that each set of wheels be "in square" with one another (i.e., their axes should be parallel).
Among the various alignment factors listed above, the present invention is concerned primarily with toe-in, toe-out in turn, offset and out of square. The importance of these measurements to tire wear will now be explained.
Wheel toe-in is a condition of the tires such that the front regions are closer together than the rear regions. Toe-out is the opposite condition. Neither is desirable in a tire rolling straight ahead, since for minimizing wear, the tires--as they roll--should be exactly parallel. Any amount of toe-in or toe-out causes increased tire wear because it causes sideways tread slipping as the vehicle moves along a road surface. Ideally, the front tires (indeed, all the tires) are precisely parallel in alignment. However, it has long been known that the front tires should be given a slight static toe-in, as measured on prior art apparatus. As the vehicle then begins to move, the resultant forces on the tires cause them to turn outward such that they roll approximately parallel. The resultant forces which cause this tendency to toe-out when rolling are due to the fact that when a vehicle moves from a stationary to a rolling attitude the "lash", or accumulated steering system tolerances, is distributed throughout the front end geometry such that the tires are slightly toed-out with respect to their static alignment.
The difference in angular orientation between the two front tires during a turn is known as "toe-out in turn". This difference in angle, which is typically about 2-3 degrees, occurs because road forces concur with the steering geometry and the inherent tendency to toe-out to cause the inside wheel on the turn to describe a smaller circle than the outside wheel. This is generally a desirable state of affairs, because it tends to foster both tires rolling through the turn rather than skidding. Problems arise, however, in properly setting toe-out in turn so as to minimize destructive forces on the tires. Toe-out in turn is largely controlled by steering arm configuration and positioning. Unless the steering arms are aligned properly, the toe-out in turn will be different in a left turn than in a right turn. According to the well-known Ackerman theory, the steering arms will yield proper, equal toe-out on turn in both directions if the steering ball joints and steering arms lie on the diagonal lines connecting the steering knuckle axes to the so-called "diamond point", which is essentially the center of the rear axle. (In a three-axle vehicle, the diamond point is between the two axles, axle- and frame-center).
A set of drivers "out of square" with its heavy load of torque can easily become a dominant force. This force can have a "bulldozer" effect (because of its tendency to move in a direction perpendicular to the axle) that sets the front of the vehicle sideways. The vehicle operator will then unknowingly react with a steering wheel change to compensate. When the torque changes, another steering wheel compensation is necessary. The operator will normally interpret this as caused by road change, so he is not put on notice that the tires are developing an unusual wear pattern, in which one wears a toe-out pattern and the other a toe-in pattern. For example, a set of drivers 1/2.degree. out of square will put the outside dual driving tires of a truck about 1/2" out of square, i.e., one 1/4" ahead and the other 1/4" behind a true vehicle transverse axis. Because of the above-described tendency of the drivers (rear tires) to go straight under heavy torque loads, the 1/2.degree. out of square condition will cause the front tires to be dragged sideways. If a truck has only a 20' span, the front tires can be dragged sideways 2" every 20' of forward travel. The destructiveness to a set of tires is obvious.
To ensure proper toe-out on turn for both turning directions, the front set of wheels must not only be in square with the rear set of wheels (assuming a two-axle vehicle), but the two sets of wheels must also be "tracking", that is, there must be no lateral offset between them. When both these conditions obtain, the sets of wheels are said to be "in diamond", and toe-out in turn will be correctly established.
Various devices and methods for measuring vehicle wheel alignment are known to those skilled in the art. For instance, the Price U.S. Pat. No. 3,805,399, issued Apr. 23, 1974, disclosed a pair of triangular frames, formed of metal rod stock. The patentee therein teaches that the triangular frames are each adapted to be urged by a pair of springs into engagement against the sidewalls of a pair of wheels. The triangular frames are used to measure toe-in in conjunction with elongate rods disposed across a vehicle. The frames are also used to support means for measuring camber, caster, and rear wheel tracking. Similarly, Knight U.S. Pat. No. 3,292,268, issued Dec. 20, 1966, also discloses a triangular frame; however, it is formed instead of square rod stock. Still another prior art device utilizes a light beam and mirror arrangement to reflect alignment readings onto a wall chart. It is designed expressly for use in a shop environment.
However, several important problems remain largely unsolved by the prior art. For instance, accuracy of measurement is at best questionable because the measuring is generally done under highly artificial conditions that would only accurately reflect true driving conditions under the most fortuitous of circumstances. Also, prior devices which utilize triangular frames that are adapted to be placed on tire sidewalls are often of such shape and height as to severely limit the types of vehicles and/or tires with which they may be used; they also are sensitive to precise placement of the suspending hooks on the tires in order to place the frames into proper engagement with the sidewalls of the tires. Additionally, most prior devices make no provisions for assessing the amount by which the various sets of wheels may be misaligned, with respect to one another, or to the vehicle frame.